Engine surge, generator

Do you know what the cause of engine surge is on my inverter generator? It is 28oo watts and with no load or eco mode off, it runs smooth. Or, a heavy load, it runs smooth. However, with a low to medium load, like a heater on low or medium, the engine revs up but never really settles into a smooth run. It’s not terrible but it’s not normal either. A bit of excess virbration from the motor and a modest amount, but very noticeable amount if surging exists.